"it means if you have a 3/4" supply to the manifold, you will see the pressure drop for 4 gpm or about 5psi. You are calculating the drop for each faucet, but they are not cumulative unless they are on the same pipe. Every section has to be calculated independently depending on the flow through it."
hj

Most places you have a flow restrictor.
The tub won't have one, a shower head does.
Like hj mentioned, you're only concerned with the individual friction loss on each line. Since you're pulling off a manifold, it's nicely balanced.

Thanks for clarifying hj's reply, however it doesn't make sense to me.

If the 3/4" supply to the manifold only has (lets say) 70PSI, then doesn't turning on the 1st listed 2.5gal/min faucet reduce the over all system pressure by 24.44PSI?
Leaving ~56PSI for the rest of the system?

These are all parallel runs, so opening each faucet will reduce system pressure, I'm just trying to get an understanding of by how much. (and if it is the amount listed in the PEX manual).

If your water pressure is 70, and you go through 100 feet of 3/4 to get to your manifold, 2.5 GPM would reduce the pressure at the manifold (is that what you mean by system?) by 1.742 PSI. That would be regardless of the size of the pipe after the manifold, as long as that pipe was carrying 2.5 GPM.

If the distance to the manifold is 25 feet, figure 1.742/4 = 0.44 PSI drop at the manifold from where you are measuring your 70 PSI.

I'm concerned about post manifold pressure. The distance from water supply to manifold is ~15ft over 3/4" pipe.

A simple example from the xls i created:
Kitchen sink is 63.56ft from the manifold, using a 3/8" pipe. According to the PEX manual (the above images) that is a 24.44PSI loss.
If I have 70PSI at the manifold, then I will not have enough pressure to supply 3 faucets.

Also, as 70-(2*24.44) = 22PSI, I will have lost enough pressure drop to notice a reduction in flow.

Please let me know if this is how it is calculated. I have to think this is routine calculations done by plumbers to determine how to adequately design PEX installations.

"Kitchen sink is 63.56ft from the manifold, using a 3/8" pipe. According to the PEX manual (the above images) that is a 24.44PSI loss."
Yes. Plenty of pressure left for faucet. If you turn on warm instead of all hot (or all cold) you will have less drop because the flow will be less in each PEX.

"If I have 70PSI at the manifold, then I will not have enough pressure to supply 3 faucets."
No. Each if each PEX is that same 63.56ft from the manifold to its faucet, and each has 2.5 GPM of flow, then each will have about 45 PSI at its faucet.

Now actually it is only a little more complex than that. Let's say there is 5o ft of 3/4 PEX to the manifold from the water meter, for example. Let's now say that there is 70 PSI at the meter rather than at the manifold. Now, with 1 faucet on, we will get 0.388 PSI drop in the 3/4 PEX which adds to the drop at the faucet. With the 3 faucets open, we will get 2.4715 PSI drop in the 3/4 PEX for a resulting pressure at each faucet of 70 - 2.4715 - 24.44 PSI which is 43.0885 PSI at each faucet.

The pressure drop to the manifold depends on the volume and the distance. But each line in a parallel (branch) off of the manifold is independent. The only common part to it is that each adds to the 3/4" line's volume. IOW, increasing the volume TO the manifold drops the overall manifold distribution lines, but what happens with pressure based on the distance of an individual branch is limited to that branch, and is only based on what volume IT is flowing...it is independent of the rest of the lines (unless they used the same line to feed multiple devices, which normally should not be done).

For example, say you have two things running, and each drawing 2.5gpm from the manifold. The 3/4" supply's volume is 5gpm with its associated pressure loss. THen, each line separately would have the loss based on its individual volume (in this case 2.5gpm and length) independent of the other branch(es) that may be running.

3/8" to an unrestricted valve may not work great (like a tub filler or say an outdoor hose bib) since they can draw a significant amount of water - the pressure loss will be noticeable. But, most things are restricted, and 3/8" would be fine.

Thank you for the replies - I'm slowly getting it.
Post manifold, each line's pressure drop is independent of each other, and according to my calculations none of my end points will drop below their restricted 2.5 gpm, as they will not go below 40PSI.

My only outstanding confusion is surely at some point having multiple faucets open will cause all of them to go below 40PSI, causing a loss in flow rate. My concern is that the 3/8"s will cause this to happen easily. 2-3 things running at once with kids is common in my house.

Until you overload the main input to the manifold, you shouldn't have a problem on individual lines...it's the cumulative volume feeding the manifold that may be an issue. Ideally, your supply is always bigger than the highest possible load, and that is determined by what's in your house. With 3/4" pex going in, that probably is not true.

You are confused. The measured pressure loss is at the END of the line at the faucet and has almost NOTHING to do with the pressure entering the pipe. You could have 150 psi at the beginning of a 1,000' pipe and only 5 psi at the end, depending on flow, but the pressure at the beginning will still be 150 psi.

Thanks hj, I was having a very hard time trying to determine that.
The supply to the manabloc is 1 inch. Supply is 40-45PSI.
The shower pressure loss is 33.42PSI.
I assume this means I will never reach the ~9gal/min flow of the shower head & 4 jets.

From what I read, when a system goes under 40PSI, you will notice reduced flow rates.

Click to expand...

See http://www.neoperl.net/en/oem/products/aerators/pressurecompensating.html for info on pressure compensating aerators.Regarding your shower, if you have a mix of hot and cold, those are traveling though separate pipes. So if your shower were to use half hot and half cold, you could get a lot of water. You must have a large house, with 75 feet of pipe from the manifold to the shower.